Re: [PATCH 09/14] iomap: Change iomap_write_begin calling convention

Darrick J. Wong
2020-10-14T16:47:54Z

On Wed, Oct 14, 2020 at 04:03:52AM +0100, Matthew Wilcox (Oracle) wrote:
> Pass (up to) the remaining length of the extent to iomap_write_begin()
> and have it return the number of bytes that will fit in the page.
> That lets us copy more bytes per call to iomap_write_begin() if the page
> cache has already allocated a THP (and will in future allow us to pass
> a hint to the page cache that it should try to allocate a larger page
> if there are none in the cache).
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> fs/iomap/buffered-io.c | 61 +++++++++++++++++++++++-------------------
> 1 file changed, 33 insertions(+), 28 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 4ef02afaedc5..397795db3ce5 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -616,14 +616,14 @@ iomap_read_page_sync(loff_t block_start, struct page *page, unsigned poff,
> return submit_bio_wait(&bio);
> }
>
> -static int
> -__iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> - struct page *page, struct iomap *srcmap)
> +static ssize_t __iomap_write_begin(struct inode *inode, loff_t pos,
> + size_t len, int flags, struct page *page, struct iomap *srcmap)
> {
> loff_t block_size = i_blocksize(inode);
> loff_t block_start = pos & ~(block_size - 1);
> loff_t block_end = (pos + len + block_size - 1) & ~(block_size - 1);
> - unsigned from = offset_in_page(pos), to = from + len;
> + size_t from = offset_in_thp(page, pos);
> + size_t to = from + len;
> size_t poff, plen;
>
> if (PageUptodate(page))
> @@ -658,12 +658,13 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> return 0;
> }
>
> -static int
> -iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> - struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
> +static ssize_t iomap_write_begin(struct inode *inode, loff_t pos, loff_t len,
> + unsigned flags, struct page **pagep, struct iomap *iomap,

loff_t len? You've been using size_t (ssize_t?) for length elsewhere,
can't return more than ssize_t, and afaik MAX_RW_COUNT will never go
larger than 2GB so I'm confused about types here...?

Mostly because my brain has been trained to think that if it sees
"size_t len" as an input parameter and a ssize_t return value, then
probably the return value is however much of @len we managed to process.

> + struct iomap *srcmap)
> {
> const struct iomap_page_ops *page_ops = iomap->page_ops;
> struct page *page;
> + size_t offset;
> int status = 0;
>
> BUG_ON(pos + len > iomap->offset + iomap->length);
> @@ -674,6 +675,8 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> return -EINTR;
>
> if (page_ops && page_ops->page_prepare) {
> + if (len > UINT_MAX)
> + len = UINT_MAX;

I'm not especially familiar with page_prepare (since it's a gfs2 thing);
why do you clamp len to UINT_MAX here?

--D

> status = page_ops->page_prepare(inode, pos, len, iomap);
> if (status)
> return status;
> @@ -685,6 +688,10 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> status = -ENOMEM;
> goto out_no_page;
> }
> + page = thp_head(page);
> + offset = offset_in_thp(page, pos);
> + if (len > thp_size(page) - offset)
> + len = thp_size(page) - offset;
>
> if (srcmap->type == IOMAP_INLINE)
> iomap_read_inline_data(inode, page, srcmap);
> @@ -694,11 +701,11 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> status = __iomap_write_begin(inode, pos, len, flags, page,
> srcmap);
>
> - if (unlikely(status))
> + if (status < 0)
> goto out_unlock;
>
> *pagep = page;
> - return 0;
> + return len;
>
> out_unlock:
> unlock_page(page);
> @@ -854,8 +861,10 @@ iomap_write_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>
> status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap,
> srcmap);
> - if (unlikely(status))
> + if (status < 0)
> break;
> + /* We may be partway through a THP */
> + offset = offset_in_thp(page, pos);
>
> if (mapping_writably_mapped(inode->i_mapping))
> flush_dcache_page(page);
> @@ -915,7 +924,6 @@ static loff_t
> iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> struct iomap *iomap, struct iomap *srcmap)
> {
> - long status = 0;
> loff_t written = 0;
>
> /* don't bother with blocks that are not shared to start with */
> @@ -926,25 +934,24 @@ iomap_unshare_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> return length;
>
> do {
> - unsigned long offset = offset_in_page(pos);
> - unsigned long bytes = min_t(loff_t, PAGE_SIZE - offset, length);
> struct page *page;
> + ssize_t bytes;
>
> - status = iomap_write_begin(inode, pos, bytes,
> + bytes = iomap_write_begin(inode, pos, length,
> IOMAP_WRITE_F_UNSHARE, &page, iomap, srcmap);
> - if (unlikely(status))
> - return status;
> + if (bytes < 0)
> + return bytes;
>
> - status = iomap_write_end(inode, pos, bytes, bytes, page, iomap,
> + bytes = iomap_write_end(inode, pos, bytes, bytes, page, iomap,
> srcmap);
> - if (WARN_ON_ONCE(status == 0))
> + if (WARN_ON_ONCE(bytes == 0))
> return -EIO;
>
> cond_resched();
>
> - pos += status;
> - written += status;
> - length -= status;
> + pos += bytes;
> + written += bytes;
> + length -= bytes;
>
> balance_dirty_pages_ratelimited(inode->i_mapping);
> } while (length);
> @@ -975,15 +982,13 @@ static s64 iomap_zero(struct inode *inode, loff_t pos, u64 length,
> struct iomap *iomap, struct iomap *srcmap)
> {
> struct page *page;
> - int status;
> - unsigned offset = offset_in_page(pos);
> - unsigned bytes = min_t(u64, PAGE_SIZE - offset, length);
> + ssize_t bytes;
>
> - status = iomap_write_begin(inode, pos, bytes, 0, &page, iomap, srcmap);
> - if (status)
> - return status;
> + bytes = iomap_write_begin(inode, pos, length, 0, &page, iomap, srcmap);
> + if (bytes < 0)
> + return bytes;
>
> - zero_user(page, offset, bytes);
> + zero_user(page, offset_in_thp(page, pos), bytes);
> mark_page_accessed(page);
>
> return iomap_write_end(inode, pos, bytes, bytes, page, iomap, srcmap);
> --
> 2.28.0
>